Actually, the ancient people of northern Europe are all related (for all intents and purposes shall be referred to as Nords); as are those of the south (the pre-migration civilisation/culture known as the Pelasgians in Roman and Greek sources and were their predecessors).
One only need to look at and compare myths, language (particularly sound and core words) and art/material culture. It is technically inaccurate in most cases to claim a connection/ difference between Celtic and Norse/Germano-northern European myth considering the absence of written records during the time of a fully realised, living breathing Celtic pagan tradition before the mass conversion to Christianity and/or interpretatio Romana or the influence of either (this includes Ireland and Wales, from which current mythic texts derive).
Despite this, for the discerning scholar, even the surviving myths, folk tales and legends recorded in works such as the Cath Maige Tuired and the Lebor Gabála Érenn of Irish myth, and the Four Branches of the Mabinogi of the Welsh; all of which were written during the Middle Ages (prior to the Norse invasions) and after the Christian conversion, adapted from a muddled and compromised oral tradition stooped in taboo. Nonetheless, they hold much insightful information.
Note, the only (potentially) unaffected Celtic cult to survive for quite some time (some claim to this day, though that is not for me to argue) is that of the Druids, whom it is noted the Danes (Vikings) respected and saw much solidarity. To add to that, it should also be noted that a similar issue details much of 'Germanic', 'Slavic' and 'Uralic' myths and at times, even known gods; due to Christianisation ~ take the Prose Edda as a prime example!
All of this does not deny close contact and back and forth influence through time, as the days of the Nords and Pelasgians stretch far into prehistory, their true myths becoming subject to mutation as they became the latter tribes we are more familiar with: Pelasgians) Greeks, Italics (including Romans, Etruscans etc.), Thracians, Iberians, Anatolians, Illyrians. Nords) Celts, Scandinavians/Norse (N. Germanic), Germans (S. Germanic ~ Anglos, Saxons, Goths etc.), Uralics, Slavs ~ These are just to name those I can remember without inquiry, and is not limited to European peoples, if one observes carefully, the world is full of allusions to the once great "core" cultures of the most distant past (Indo-European is a fallacy, derived from inaccurate reconstruction based on a misunderstanding of lingua franca languages and the koine which may develop from them such as Sanskrit, as being purely original and therefore related via common ancestry).